In
1677 Edmond Halley (of Halley's comet fame) began to think about
using transits to find the Earth-Sun distance (the astronomical
unit or AU). He finally developed a workable method: astronomers
at two widely separated places on Earth measure the exact time it
takes for Venus to cross the Sun. Knowing the difference in the
times of passage and the distance between the observers, Halley
thought astronomers could calculate how far Venus is from Earth.
Then, using Kepler's laws, they could determine the AU.

The
French astronomer Joseph-Nicolas Delisle improved on Halley's method.
If the two observers know their exact positions on Earth, he argued,
they only need to record the moment when the edge of Venus lines
up with the edge of the Sun. Even if clouds prevent them from seeing
the entire transit, they will still have valid measurements.

Mobilizing
for the 1761 and 1769 Transits

All
over Europe and North America, astronomers prepared for the 1761
transit of Venus, vying to be the first to determine the true Earth-Sun
distance. They would need to observe it from widely separated places
on Earth, covering several sites in each area, in case bad weather
(or rotten luck) affected some observations.

The
best place to see the entire 1761 transit was India and the East
Indies. The most distant site that would complement those locations
was Siberia. Accordingly, French and British academies of science
sent astronomers to both regions and other distant points around
the world.

Those
expeditions helped astronomers better prepare for the 1769 transit
when Russia, Sweden, and Denmark joined in observations. The 1769
transit was observable from America, so many astronomers watched
it in places like Boston, Philadelphia, and Providence. Click
here to see a map representing past expeditions.

To
the Ends of the Earth

The
1761 and 1769 transit-of-Venus expeditions suffered great trials
and achieved moderate success.

Due
to the expense of mounting major expeditions, a great deal of additional
scientific activity was included. A number of expeditions, especially
the well-known voyage of Captain Cook, made many natural history
and anthropological observations and brought back specimens and
artifacts. The voyage signaled a new age of scientific exploration.

More
than 120 transit observations were recorded in 1761, but most were
of poor quality, due to optical problems and inexperienced observers.
In 1769 more than 150 observations were recorded - with no better
results. Nevertheless, the range of values for the Earth-Sun distance
was narrowing to between 94 and 96 million miles. Astronomers would
have to wait another century for the next transit of Venus and their
next attempt at finding the precise answer.

The
Perils of Chasing Venus

Traveling
across the world could be grueling in the 1700s. Most of the transit
expeditions suffered hardships, but two French astronomers were
particularly unfortunate.

Guillaume
Le Gentil de la Galaisière traveled to India to observe the
1761 transit from the French colony of Pondicherry. Before he arrived,
the British captured the colony, forcing Le Gentil to return to
the French island of Mauritius. He did not get there in time to
observe the transit. Rather than return home, he studied the Indian
Ocean cultures, planning to watch the 1769 transit from Manila in
the Philippines. But he was ordered back to Pondicherry, again under
French control. While Manila had beautiful weather during the transit,
it rained in Pondicherry. Le Gentil missed both transits. When he
returned home in 1771, he learned that he had been declared dead.
After many expensive legal battles, Le Gentil managed to prove he
was actually alive.

For
the 1769 transit, Jean Chappe d'Auteroche led a party to the southern
tip of Baja California. They observed the transit under ideal conditions,
but a few days later an epidemic struck the area, killing three-fourths
of the local population and everyone in the expedition except for
Chappe and Pauly, his engineer. Chappe lingered for a short while
but died before leaving Baja. Pauly managed to get back to France
with all of the observation records.